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How is plastic recycled?
In the United States 75
billion pounds of plastic are produced every year, unfortunately the
majority of this plastic ends up in landfills. When plastic is dumped
into landfills the decomposition process can take anywhere from 10 to 30
years. Recycling has therefore become a reasonable solution to the
landfill problem.
There are five factors that
are necessary in order for the recycling of plastic to be a successful
process. First, the supply of used plastic has to be of a large
quantity. This large quantity of plastic is collected at certain areas,
which is the second step. Once the plastic is collected, the sorting and
separating process begins; this is the third step in the process. The
sorting and separating process depends upon the type of polymers that
make up the plastic. Plastic products are given codes to help the
sorting and separating process. The fourth step in plastic recycling is
reprocessing. The reprocessing of polymers includes the melting process,
the melting process can be accomplished if the polymers have not been
widely cross-linked with any synthetics. If the cross-linking of
polymers contain too many synthetics, the polymers will be difficult to
stretch and less pliable. The final step is the manufacturing of the
melted plastic into new products.
The codes on plastic
recyclable containers are what help most in the sorting and separating
process. The six categories of plastics are separated into two areas:
polyethelyne plastics and polymer plastics. The polyethelyne plastics
are labeled HDPE, for high density polyethelyne; or LDPE, for low
density polyethelyne. The four polymer plastics that are recycled
include polyvinyl chloride, labeled V; polystyrene, labeled PS;
polypropylene, labeled PP; and polyethylene terephthalate, labeled PETE.
These names and labels can seem confusing, but they are a necessity in
the recycling process.
There are four types of
recycling processes that usually occur: primary, secondary, tertiary,
and quaternary. The primary recycling process is recycling materials and
products that contain similar features of the original product. This
process is only feasible with semi-clean industrial scrap plastics,
therefore this process is not widely used. Secondary recycling allows
for a higher mixture of combination levels in plastics. When the
secondary process of recycling is used it creates products such as
fenceposts and any products that can be used in the substitution of
wood, concrete, and metal. The low mechanical properties of these types
of plastics are the reason why the above products are created. Tertiary
recycling is occurring more and more today because of the need to adapt
to the high levels of waste contamination. The actual process involves
producing basic chemicals and fuels from plastic. The last form of
recycling is the quarternary process. This quarternary process uses the
energy from plastic by burning. This process is the most common and
widely used in recycling. The reason this process is widely used is
because of the high heat content of most plastics. Most incinerators
used in the process can reach temperatures as high as 900 to 1000
degrees Celsius. For the sake of the environment the new techniques
being used with the incinerators have decreased the amount of air
pollutants being released.
The use of incineration in
the quarternary process is most beneficial because through the high
temperature heating process the incoming waste is reduced by 80% in
weight and 90% in volume. The materials left over form this process are
then placed in landfills.
Why Worry About Recycling
Plastics?
A current promotional
program sponsored by the plastics industry emphasizes the positive
contributions that plastics make. And claims listed in those
advertisements are accurate.
But as shown in the article
on the preceding page, the largest single use for plastics is packaging.
Because packaging has a short lifespan, it makes up a large portion of
the plastics waste stream. But where does that “waste stream” lead?
In general, the
Environmental Protection Agency says that in the early 1990s about 80
percent of all municipal solid waste was sent to landfills, 10 percent
was incinerated and 10 percent was recycled. While more and more plastic
is being recycled, the EPA estimates that plastics make up about 20
percent of the solid waste that is landfilled.
Most consumers think that
the slow degradation of plastics is the primary reason that plastics
should be recycled. However, research has shown that other waste, such
as paper, wood and food wastes, also degrade very slowly in landfills.
The more serious problem
with plastic waste concerns the additives contained in plastics. These
additives include colorants, stabilizers and plasticizers that may
include toxic components such as lead and cadmium. Studies indicate that
plastics contribute 28 percent of all cadmium in municipal solid waste
and about 2 percent of all lead. Researchers don’t know whether these
and other plastic additives contribute significantly to products leached
from municipal landfills.
How toxic are plastics that
are burned? Researchers don’t know that, either. Plastics that contain
heavy-metal-based additives may also contribute to the metal content of
incinerator ash. The EPA is looking for substitutes for lead- and
cadmium-based additives.
One additional concern
relates to use of petroleum products. All plastics began their lives as
petroleum. By increasing plastics recycling, scientists and engineers
are able to reduce dependence on petroleum.
First Things First:
Sorting
Before plastic waste can be
converted into new products, the various types of plastics must be
separated. Initially plastic reclamation companies relied on manual
sorting — either by consumers themselves or by paid workers — but manual
sorting is considered too unreliable and too expensive.
At least two organizations
have developed systems for automated plastics sorting. National Recovery
Technologies received the EPA’s Small Business of the Year National
Award in 1991 for its efforts in developing and marketing a high-speed,
automated system that efficiently separates vinyl containers (those
marked #3) from mixtures of whole or crushed post-consumer plastic
containers. NRT says that the presence of chlorine atoms within vinyl
resins triggers a computer-timed air burst that separates vinyl
containers from the mixed plastic stream. The company also developed a
system that optically scans mixed plastics to separate PET soda bottles
from HDPE milk jugs, green PET
from clear PET, as well as other specifications.
Sandia National
Laboratories, which works with the U.S. Department of Energy, has
designed a device to classify plastic waste into one of the seven
plastics categories. Near-infrared light is used to distinguish one
plastic from another using the vibrational characteristics unique to
each. Sandia engineers report that the device can classify many types of
plastics with a success rate of 98 to 100 percent. The laboratory has
issued a license for commercial development of this new device.
Old ABS Phone Housings
Recycled Into Innovative Mounting Panels
Seeking new uses for
recycled plastic from old telephones, AT&T Bell Laboratories engineers
are remolding discarded phone housings into mounting panels for AT&T’s
business telephone systems and improving service to business customers
as well.
“Until now, when a telephone
reached the end of its life, AT&T would sell the plastic to a recycler
who would grind it up and resell it into the secondary market, where it
was made into products ranging from tape cassettes to park benches,”
said Werner Glantschnig, a member of Bell Laboratories technical staff
and the project’s leader.
“However, we wanted to see
if we could close the loop ourselves and re-use these millions of pounds
of ‘ABS plastic flake’ in a way that makes both environmental and
business sense.”
ABS — or
acrylonitrile-butadiene-styrene — plastic flake can’t be made into new
telephones because colors change during re-melting, and the plastic
loses the smooth, glossy finish AT&T requires for its phone housings.
“When we mold the ABS into
telephone system mounting panels, the colors disperse nicely into a
uniform gray and the finished product meets all of our requirements,”
said Louis D’Anjou, another Bell Laboratories engineer and the panel’s
designer.
With these ABS panels,
AT&T’s supply centers can now assemble and test business telephone
systems before delivery to the customer’s premises. Previously, these
panels had to be custom-made from plywood and the system assembled and
tested at the customer’s location. In addition to reducing the use of
wood, the new method is far more efficient, reducing both the time and
cost of installation.
AT&T engineers and designers
are looking into other possible uses for the ABS plastic flake,
including spools for copper and fiber optic telephone cables.
“This is encouraging
evidence that environmental awareness and the concept of ‘design for
environment’ are spreading through the AT&T design community,” said John
C. Borum, AT&T environment and safety engineering vice president. “Our
goal is to remain in the vanguard of environmentally responsible
corporations by recycling as many of our products as we can.”
Junk Car Seats Lead To
New Technology
Throughout the environmental
movement, researchers are concerned with products that are discarded in
large quantities. Junk cars are one such product. But junk cars pose
unique problems because of the combination of products used in
automaking. Used metals from junk cars have long been recycled, and now
many researchers are turning their attention to other auto components.
The Center for Excellence in
Polymer Science and Engineering at the Illinois Institute of Technology
has focused one project on the 400 million pounds of polyurethane foam
scrapped each year from junk cars. IIT has patented a solid state sheer
extrusion process and apparatus that could be used to recycle that
waste, along with a broad range of other polymer wastes and rubbers.
IIT’s system, known by the
acronym SSSE, pulverizes polymeric material, producing fine powders that
have numerous applications for industry. The advantage of SSSE is that
it can be applied economically to many types of natural and synthetic
polymer wastes. The Center for Excellence in Polymer Science and
Engineering points out that many recycling processes developed to date
have been limited to certain types of waste. Most processes have not
been economical, especially in the amount of energy needed, they add,
and the reclaimed materials have not been produced in forms that are
needed and usable for re-manufacturing.
The SSSE technology
has been optioned to a New York firm for possible development. More
information is available through IIT’s Office of Public Relations,
(312) 567-3104.
Research Into Chemical
Recycling Could Open New Opportunities
The U.S. Department of
Energy conducts on-going research on plastics recycling. This report
highlights new approaches to chemical recycling. Recycling of plastics
can be costly and difficult because of constraints on waste
contamination and inadequate separation prior to recycling. Chemical
recycling could remove some of those restraints.
Pyrolysis and hydrolysis are
two processes that have shown promise in the recovery of basic chemicals
and fuels from waste plastics. Pyrolysis is a process in which plastic
wastes are heated in the absence of oxygen in a closed chamber. The
products of pyrolysis may be used as a chemical feedstock or fuel.
Hydrolysis decomposes plastic wastes through a series of chemical
reactions.
Research sponsored by the
U.S. Department of Energy’s Office of Industrial Technologies at the
National Renewable Energy Laboratory has led to the development of a new
process based on the pyrolysis of certain waste streams. This process
retrieves monomers, the basic building blocks of a polymer, and
high-value chemicals that are sufficiently pure to use in making new
plastics. The advantage of this process is that the waste plastics do
not have to be separated ahead of time, thereby eliminating a
labor-intensive step in current processes. It also will reduce the cost
of the monomers and chemicals and will reduce consumption of petroleum,
the source of chemical feedstocks used to produce plastics.
In the new process, monomers
and high-value chemicals are retrieved from manufacturing or
post-consumer wastes through sequential pyrolysis. The reaction products
undergo detailed chemical analysis to determine conditions that allow
control of pyrolysis reactions. This allows the design of a process to
collect the desired products in high yields, reducing requirements for
subsequent separation and purification of the target product. NREL has
filed patent applications to cover the process for a total of seven
mixed plastic waste streams.
For example, NREL has
demonstrated the new process of waste carpet recycling. Caprolactam, the
valuable monomer of Nylon 6 used in about half of all carpet fibers, can
be isolated with yields of 85 percent. This can be done without
separating the nylon from the backing material.
An economic evaluation of
recycling caprolactam performed by an independent economic firm shows
the applications to be promising. Those findings project that a
commercial-size plant recycling 100 million pounds of waste carpet could
produce high-grade caprolactam for about 15-50 cents per pound. The
chemical currently sells for 90 cents to $1 per pound. In other words,
recycling caprolactam could reduce its cost by 50 percent or more.
Other applications of the
chemical recycling process include recovering terephthalic acid from
polyethylene terephthalate, or PET, in mixed plastic bottles and
recovering styrene from mixed residential plastics. PET recycling does
not have as favorable economics as the polyurethane application because
of the lower value of plastic bottles but the potential volume of the
waste stream is very large. Researchers estimate that 900 million pounds
of thermoplastic polyester resin, of which PET is a major component,
could be recycled each year.
Researchers are expanding
the technology base for the chemical recycling process and are
identifying new, promising applications for specific waste streams.
Experiments are currently underway using engineering-scale reactors to
confirm process reactions and to refine operating conditions.
Do-It-Yourself Plastic:
Gloop
Here’s a simple activity to
share with students to help them understand some characteristics of
plastics and other polymers. It was developed for the Sandia National
Laboratory “Science and Math Carnival.” This activity is suggested for
students in third through eighth grade.
Gluep is a polymer made from
borax (sodium tetraborate) and white glue. Each of these materials is a
polymer already. Glue is a mixture of polyvinyl acetate and polyvinyl
alcohol. Borax forms long borate chains in an aqueous solution. When the
two materials are mixed together and the mixture is kneaded by hand,
crosslinking of the polymer chains occurs as a result of hydrogen
bonding with water molecules which links the two polymer chains. The
physical properties of the mixture are quite different from the
properties of the individual compounds. The resulting Gluep is a
semi-solid plastic-like material.
Materials
4 percent borax
solution (1/4 cup of borax dissolved in 1 quart of tap water)
White glue mixture (50:50 mixture of glue and water, mixed well)
Food coloring
Ziploc plastic bags
Procedure
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Pour 15 ml (1 tablespoon)
of the borax solution into the bag.
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Add three drops of food coloring.
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Add 60 ml (4 tablespoons) of the white glue mixture.
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Zip the plastic bag tight.
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Knead thoroughly until the color is uniform and water is no longer
visible. Consistency should be reached within 10 minutes.
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Remove Gluep from the bag by turning the bag inside out and rubbing
the Gluep from the sides.
-
Store the Gluep in the plastic bag.
Questions
How is Gluep like a solid?
Like a liquid? What happens if you leave Gluep out of the bag? What
happens if you freeze it?
Extension
Try other experiments with
the Gluep. What happens when extra borax solution is added? What happens
when extra glue is added? What happens if a base or acid is added during
mixing? After the Gluep has hardened?
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